Procedure 4 Large Structures Construction Stage Analysis)
7.4.4 Parameters for Modelling Creep & Shrinkage
7.4.4.2 System parameters for creep and shrinkage
Other parameters influencing the creep and shrinkage behaviour are related to the fol- lowing:
• The environment of the construction site, especially the climatic conditions.
• The cross-section geometry and reinforcement content
• The casting time.
All these parameters are element parameters, related to the structural elements and constant over the element length. This applies also to the cross-section parameters de- scribed below, which are average values related to a characteristic intermediate cross- section if the cross-section is not constant over the element.
Climatic parameters:
RH(%) Relative humidity at the construction site for the element being considered (time independent average value) in [%].
TMP Average global temperature for the element being considered.
The climatic parameters used in RM2000 are the average environmental temperature TMP and the relative humidity RH at the construction site. Although they are generally the same for all elements of a structure, they can be entered in "STRUCTURE
#ELEMENT !TIME as element attributes together with the parameters related to cast- ing or activation time respectively. Different values may, for instance, be used for the parts of the structure exposed to the air, for parts beneath the ground surface and for parts under water.
The specified values are, however, constants and may not be specified as time depend- ent functions taking into account the different seasons during construction time. An ap- proximate approach for considering the season influence could be in forming weighted average values related to the casting season, taking into account the higher creep rate in the first months.
The values RH and TMP are assigned internally to the variables TMP and RH. These variables may also be used for other user defined Variable definitions.
Element (Cross-Section) parameters:
Ax Cross Section Area
U Outside perimeter exposed to drying
UIN Inside perimeter (of a hollow box cross-section) RPR Stiffness ratio steel/concrete ((As*Es)/(Ac*Ec))
These parameters are used for the calculation of the “notional member size” or “no- tional thickness”, which influences the time development of the creep and shrinkage functions which themselves are dependent on the speed of drying.
Note: They normally do not influence the final value of the C&S coefficients.
These parameters (except RPR) need not and can not be entered by the user as they are automatically calculated for the cross sections of the elements that can creep. The aver- age values will be taken if the cross sections at the element begin and end differ.
These parameters may however be reset by the user in "PROPERTIES #VARIABLE
RPR – Stiffness ratio [-]
The element parameter RPR (defined in "STRUCTURE #ELEMENT !TIME) char- acterizes the ratio between the normal force stiffness of the reinforcement and that of the concrete cross-section ((As*Es)/(Ac*Ec)). It is used in some creep laws (e.g.
Hongkong Standard and British Standard) for determining the creep coefficient and/or the shrinkage coefficient.
However, it does normally not affect very much the resultant values of the coefficients, therefore it is mostly sufficient to neglect the influence of the reinforcement (RPR=0), or to enter a rough approximate value.
The parameter RPR is assigned internally to the variable RPR to be used for further user defined variable definitions.
Usage and dependencies for WCR in the standard creep models:
Model Creep Shrinkage
AS96 - -
BS5400, HS5400 + +
CEB78, CEB90, RSM90 - -
DIN1045, OE4750 - -
Norway, Hungarian + +
(-)...not used (+)...used
Time parameters:
The below described time parameters are in all creep and shrinkage models used for calculating the values of the creep and shrinkage coefficients.
AGE (Day) Age of the concrete at the element activation time (difference between the time of casting and the time of activation) measured in days.
tS (Day) Start time for shrinkage relative to the casting time (i.e it is not necessar- ily relative to the activation time) measured in days.
The time t=0 of the creep model is the time that the concrete for that element is cast (casting time). The creep curve diagram starts at time = 0 (days) even though the ele- ment only starts to creep when load is applied to it. – i.e. at activation time – this is when the formwork is removed or the pre-stressing is applied (self weight will act on the concrete plus pre-stressing and construction loading where applicable.)
The parameter AGE is used to locate the casting time on the global time axis. Therefore AGE is the time (in days) on the creep curve before the element activation time TG,A.
Some C&S models allow the starting time of shrinkage to be different from the starting time of creep. The starting time for shrinkage is defined by the parameter tS.
tS is the age of the concrete when the shrinkage process starts, i.e. shrinkage will start at the time TG,A-AGE+tS =TS (in days).